1. Field of the Invention
The invention relates to a method and system for power amplification, more particularly to a method and system for low-distortion power amplification in which the effects of both phase noise due to non-symmetrical phase shift and internal system noise can be effectively minimized.
2. Description of the Related Art
In a conventional digital power amplifier, an input signal is amplified with the use of a power switching circuit. Two types of power switching circuits are available in the art, namely the half-bridge switching circuit and the H-bridge switching circuit. Transistors are commonly used as switching elements for accomplishing the switching operation. Assuming that transistors are ideal elements, when a pulse-width modulated (PWM) input signal of the digital power amplifier undergoes switching by the switching elements of the power switching circuit, the PWM input signal should actually be amplified. High-frequency components are then eliminated with the use of a low-pass filter such that a complete low-frequency amplified signal can be obtained from an output end of the digital power amplifier. However, the complete signal as such is unlikely to be realized by an actual circuit due to the generation of different types of noise in the digital power amplifier. For example, the power supply of the digital power amplifier will normally generate ripples. In addition, the transistors that serve as the switching elements are non-ideal elements, and when the switching elements switch alternately between ON and OFF states during signal amplification, the ON and OFF switching cycles will generally not match completely. Furthermore, the load characteristics also introduce noise into the digital power amplifier. There is thus severe distortion in the amplified output signal of the digital power amplifier.
A pulse edge delay technique has been proposed heretofore to eliminate the noise introduced by the aforesaid factors. In PCT International Publication Number WO 98/44626, there is disclosed a digital power amplifier that includes a correction unit, a power switching unit and an error processing unit. The correction unit is coupled to the power switching unit and the error processing unit. The correction unit receives a pulse-modulated input and, according to an error signal from the error processing unit, delays the edges of pulses of the pulse-modulated input. After corresponding adjustment of the pulse width, the correction unit provides a corrected pulse-modulated signal to the power switching unit. The power switching unit amplifies the corrected pulse-modulated signal to generate an output signal. The output signal is fed back to the error processing unit, which compares the output signal with the pulse-modulated input to result in a new error signal that is sent to the correction unit for adjusting the pulse width of a succeeding pulse of the pulse-modulated input.
In the pulse edge delay method employed in the aforesaid publication, the width of each pulse of the pulse-modulated input is adjusted according to the error signal so as to eliminate the effect of internal system noise in the digital power amplifier. However, a phase noise is generated in the pulse edge delay method of the aforesaid publication. The phase noise is actually harmonic noise that is introduced due to the non-constant time difference formed between the center of each pulse of the pulse-modulated input and the center of the corresponding pulse of the corrected pulse-modulated signal.
Therefore, the main object of the present invention is to provide a method and system for low-distortion power amplification in which the effects of both phase noise and internal system noise can be effectively minimized so that a low-distortion amplified signal output can be obtained.
According to one aspect of the invention, a method for low-distortion power amplification comprises the steps of:
generating an error signal by comparing a feedback signal obtained from an amplified output signal at an output terminal of a power switching stage with one of an input pulse-width modulated signal from a pulse-width modulation signal generator and a corrected pulse-width modulated signal at an input terminal of the power switching stage; and
correcting each pulse of the input pulse-width modulated signal from the pulse-width modulation signal generator to result in the corrected pulse-width modulated signal that is provided to the input terminal of the power switching stage, said each pulse of the input pulse-width modulated signal being corrected by adjusting the phase thereof such that said each pulse of the input pulse-width modulated signal has a center that forms a predetermined time difference with a center of a corresponding pulse of the corrected pulse-width modulated signal, and by adjusting the width thereof according to the error signal.
According to another aspect of the invention, a system for low-distortion power amplification comprises a pulse-width modulation signal generator, a power switching stage, an error signal generator, and a correction device.
The pulse-width modulation signal generator generates an input pulse-width modulated signal.
The power switching stage has an input terminal to receive a corrected pulse-width modulated signal, and an output terminal from which an amplified output signal is obtained.
The error signal generator includes a feedback signal generator, a comparator, and a processing unit. The feedback signal generator is connected to the output terminal of the power switching stage and is operable so as to generate a feedback signal from the amplified output signal at the output terminal of the power switching stage. The comparator is connected to the feedback signal generator and one of the pulse-width modulation signal generator and the input terminal of the power switching stage. The comparator compares the feedback signal with one of the input pulse-width modulated signal and the corrected pulse-width modulated signal to result in a difference signal. The processing unit is connected to the comparator for processing the difference signal to remove high frequency components therefrom so as to result in an error signal.
The correction device is connected to the pulse-width modulation signal generator, the processing unit and the input terminal of the power switching stage. The correction device corrects each pulse of the input pulse-width modulated signal from the pulse-width modulation signal generator to result in the corrected pulse-width modulated signal that is provided to the input terminal of the power switching stage. The correction device corrects said each pulse of the input pulse-width modulated signal by adjusting the phase thereof such that said each pulse of the input pulse-width modulated signal has a center that forms a predetermined time difference with a center of a corresponding pulse of the corrected pulse-width modulated signal, and by adjusting the width thereof according to the error signal from the processing unit.